Light-emitting chemical reactions (chemiluminescence, CL) and light-emitting biological reactions (bioluminescence, BL) have a diverse range of analytical applications. Advantages of CL and BL assays include high sensitivity due to enzyme amplification, rapid signal generated in a few seconds, and assays do not need an external excitation light source. In many situations, these procedures are replacing the use of radioactive nuclides. As luminescent agents have become more efficient, many more studies are making use of luminescence assays as analytical tools. Chemiluminescent substrates, such as dioxetane, luminol, acridinium ester and hydrazide, have been developed. These compounds are catalyzed by hydrolytic enzyme and the resulting products emit light. Bioluminescent reactions are generally more efficient than chemiluminescence. BL has traditionally been associated with firefly luciferase. AquaLite (SeaLite Sciences, Ga.) is a recombinant form of a photoprotein from jellyfish. It can be triggered to produce all of its light in a single step within a few seconds. CL and BL methods have been developed for many enzyme labels (alkaline phosphatase, galactosidase, horseradish peroxidase, etc.). The enzymes are conjugated to the secondary antibody or analytes for subsequent substrate reactions. In a typical sandwich immunoassay, the analyte is sandwiched between the antibody conjugate and immobilized probes. The luminescence intensity at any time is a direct measure of the concentration of enzyme conjugate or analyte for positive identification. The newly developed dioxetane offers a detection sensitivity of 600 molecules (10−21 mole), making it several orders more sensitive than the fluorescence-based assay. Rather than a luminescent species being directly attached to a target analyte or to its binding partner, an enzyme is used to catalyze a luminescent reaction. The catalytic turnover ability of the enzyme allows thousands of potentially luminescent reactions to occur per second as long as sufficient substrate is present. Less than 10−21 mole of alkaline phosphatase can be detected in solutions using dioxetane-based compound, such as Lumi-PPD and Lumi-PS-1 of Lumigen Inc. (Southfield, Mich.). When Lumi-PPD is added to a microwell containing alkaline phosphatase, the resulting chemiluminescence reaches a maximum after 5-10 minutes and remains constant for more than an hour. Various luminescence detection devices (for example, luminometers) are commercially available. U.S. Pat. No. 6,018,387 discloses a method with an optical reflector to increase luminescence collection efficiency in a luminometer system and U.S. Pat. No. 6,187,267 addresses a free-space luminometer with a confocal optical array to detect an array of samples. No microfluidic system has been disclosed in luminometer patents.
Numerous immunoassay and nucleic acid assays, such as RIA, ELISA, fluorescence, polarization, and chemiluminescence, have been developed to varying degrees based on instrumental or visual inspection of results. Three common methods include: First, the dip-stick (membrane strip): the pregnancy test is a typical example. While such tests have proven useful in obtaining qualitative results based on the determination of analyte, they are not very useful for making quantitative measurement, especially with whole blood or in disease diagnoses requiring high sensitivity. Second, the modular cluster systems can run up to several thousands of samples per day. These modular workstations with robotic liquid handlers have the capability of running 200-800 tests per hour in hospitals and clinical laboratories. They often operate in a batch mode after a large collection of sample. These systems are too large to be used for near-patient-site tests. Third type is the point-of-care testing (POCT) device. Many hospitals have started to set up POCT training program for nurses and clinicians. I-Stat has a hand-held POCT device, described in U.S. Pat. No. 5,096,669, with a small cartridge for measuring the “chemical” components (blood gas, pH, sodium, potassium, etc.) in blood samples. The system based on one-step conductivity measurement and is very easy to use. Therefore, it is desirable to have microfluidic-based POCT for immunoassay (bacteria, virus, protein, hormone, cell receptor, etc.) and nucleic acid analysis (gene mutation, gene expression, oligonucleotide, DNA, RNA, single nucleotide polymorphism, etc.). However, immunoassay and nucleic acid assays often require multiple-step of reactions. Automating multiple-step reactions in a microfluidic biochip platform have been very challenging.
Nucleic acid analysis has been known to produce very sensitive testing results. The quantity of the analyte in the sample is very small. Polymerase Chain Reaction (PCR), U.S. Pat. No. 5,229,297, technique provides a high level of sensitivity, but it takes 20-40 thermal cycles and 2-4 hours to prepare the sample and amplify nucleic acid samples. Most DNA chips are based on PCR amplification, it is too time consuming for POCT application. “Laboratory on a chip” technologies, as disclosed in U.S. Pat. No. 6,238,538 and No. 6,270,641, are based on the electro-osmosis mechanism for nucleic acid electrophoresis. U.S. Pat. No. 6,271,042 utilizes a biochip structure with CCD camera for fluorescence detection. U.S. Pat. No. 6,215,894 discloses a method to process the biochip image. U.S. Pat. No. 6,242,246 describes a method to detect nucleic acids in a biochip platform based on nucleotide affinity reactions. None of the microfluidic biochip is based on non-PCR chemiluminescence detection.
CL and BL are used to detect nucleic acids using the binding nature of double strand oligonucleotides. Oligonucleotide probes hybridize with samples or conjugated (biotinylated) oligonucleotides on solid surfaces. The biotinylated oligonucleotides can easily bind to streptavidin-enzyme conjugate with very high affinity. Consequently, enzyme conjugates can trigger luminescence substrate to produce light as that occurred in chemiluminescent immunoassay. Instead the sensitivity of the reaction is judged by amplification of a signal. Since there are no thermal cycles involved in DNA analysis, the consequence of cross-contamination between samples is reduced when compared to PCR. The luminescence is also applied to the recently developed branched DNA (bDNA) assay. The concept of branched DNA is derived from the assembly of structural components in all biological systems. DNA possesses a well-known structure, which is invariant to sequence. The predictability of the association of DNA's cohesive ends, is the basis for genetic engineering endeavors that underlie modern molecular biology. DNA molecules with single-strand “sticky” ends are used to direct the assembly of individual fragments in a particular order. The predictability of sticky-ended ligation extends to the geometry of the product. Therefore, it is possible to design oligonucleotide sequences of DNA that will self-assemble to form stable branched complexes. A DNA branch junction has been built with as many as six double helical arms. The bDNA assay requires minimal sample preparation, allowing for rapid or point-of-care testing. Based on the double amplification principle, branched DNA multimer and enzyme chemiluminescence, the biochip provides microfluidics for all necessary nucleic acid chemistries. The advantage of the microfluidic biochip system is to automate the delivery of samples, hybridized amplifiers, enzyme conjugates, chemiluminescence substrates, and washing solutions to a reaction site in a predetermined order. After hybridization of enzyme-labeled probes with the amplifiers, the oligonucleotide-probe complex is detected by use of a chemiluminescence substrate. Branched DNA kit, commercialized by Bayer Diagnostics, is a clear deviation from PCR amplification technologies because the target nucleic acid is not replicated. The combination of bDNA luminescence and automated microfluidic biochip not only provides high sensitivity, but also offers simple and rapid testing.
The establishment of miniaturized microfluidic systems has brought to the immediate horizon the possibility of point-of-care analysis, to be achieved by extremely fast, hand-held, or portable laboratories. Many firms and research institutes are actively engaged in developing microfluidics and microarray technologies. Although significant efforts have been put into the development, most biochips are not self-contained; therefore, external reservoirs are used to store the reagents. When a biochip is connected to external reservoirs, it requires pumps and tubes. The resulting system becomes too large and too complex for POCT applications.
Prior to this invention, POCT devices are commonly lack of sensitivity. No system demonstrates the same performance: sensitivity, reliability, and precision as large stand-alone or modular systems. In combination of microactuator, microfluidic biochip, and highly sensitive luminescence detection mechanism, POCT devices can achieve same performance as the large workstations.